Next generation wireless communication systems are expected to be able to transmit multimedia data of high quality at a high speed by using limited radio resources. To achieve this in a wireless channel having a limited bandwidth, inter-symbol interference and frequency selective fading, which occur during high-speed transmission, have to be overcome while maximizing spectral efficiency.
In general, a wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available radio resources. Examples of the radio resources include time, frequency, code/sequence, transmit (Tx) power, etc. Examples of the multiple access system include a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, a multi carrier frequency division multiple access (MC-FDMA) system, etc. The radio resource is time in the TDMA system, frequency in the FDMA system, code in the CDMA system, and subcarriers and time in the OFDMA system.
The SC-FDMA has almost the same complexity as the OFDMA, and has a lower peak-to-average power ratio (PAPR) due to a single carrier property. Since it is advantageous for a user equipment (UE) to have a low PAPR in terms of Tx power efficiency, the SC-FDMA is adopted for uplink transmission in a 3rd generation partnership project (3GPP) long term evolution (LTE) as disclosed in section 5 of the 3GPP TS 36.211 V8.2.0 (2008 March) “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8)”.
A closed-loop transmission scheme using a channel condition between a base station (BS) and a UE is introduced to improve capability of the wireless communication system. An adaptive modulation and coding (AMC) scheme is a technique in which the BS regulates a modulation and coding scheme (MCS) by using feedback channel state information to increase link capability.
The UE reports a well-known control signal such as a channel quality indicator (CQI) to the BS, and thus the BS can know a downlink channel state. The BS may perform frequency selective scheduling by receiving the downlink channel state from each UE. If the frequency selective scheduling is also performed in uplink, the BS needs to know an uplink channel state.
An uplink channel state is measured by using a reference signal. The reference signal is known both to the BS and the UE, and is also referred to as a pilot. The reference signal is classified into two types, i.e., a demodulation reference signal and a sounding reference signal (SRS). The demodulation reference signal is used in channel estimation for data demodulation. The SRS is used in uplink scheduling. When the UE transmits the SRS to the BS, the BS estimates an uplink channel by using the received SRS. The estimated uplink channel is used in uplink scheduling.
Meanwhile, a multiple antenna system is a communication system using a plurality of transmit (Tx)/receive (Rx) antennas. The multiple antenna system can linearly increase a channel capacity without additional increase of a frequency bandwidth as the number of Tx/Rx antennas increases. In a case where communication is achieved using the multiple antenna system, it is not effective to directly use the conventional communication method using a single antenna. Accordingly, there is a need for a method to be used when a UE intends to transmit an SRS by using multiple antennas.